Honda MT125

The Honda MT125R was produced for the U.S. market in the years 1977–1978. It was a production Grand Prix motorcycle racing roadracer designed for closed-course competition roadracing. The MT125R were produced by Honda Racing Service Center Honda Racing Corporation RSC and made available to the general public through the American Honda Motorcycle dealer network.

For many champion motorcycle roadracers, the road to the top started on a small engine capacity motorcycle. Top roadracers like Randy Mamola got their start on a 125cc GP racer. Others like Angel Neito spent their entire careers racing in the 50cc, 80cc and 125cc classes.

1977 - MT125R

Product code: 325

Serial number ranges

• Frame: MT125R2f-1081~
• Engine: MT125R2E-1081~

Features

• Engine - 123cc 2-stroke air-cooled single cylinder, 6-speed, power 26HP
• Chassis - fiberglass tank, fairing, fender and seat cowl
• Other - tachometer, mechanical front disk brake, and rear cable operated drum brake

1978 - MT125R

Product code: 325

Serial number ranges

• Frame: MT125R2f-1591~
• Engine: MT125R2E-1591~

Features

• Engine - 123cc 2-stroke air-cooled single cylinder, 6-speed, (1978 change), power 28 (1978 change)

Expansion chamber redesigned

• Chassis - fiberglass tank, fairing, fender and seat cowl
• Other - tachometer, (1978 change) - hydraulic front disk brake, rear cable operated drum brake

Overview

Good examples of this model will have the original "RSC" tachometer, factory prop stand which lifts at the footrests, owners/service manual.

A liquid cooled kit was available from RSC which included: liquid cooled cylinder, cylinder head, water pump, hoses, hardware, temperature gauge and radiator. This option is fairly rare.

The MT125R shares many parts with the CR125 engine. The CR versions were motocross bikes with similar specifications. One notable difference was the shift pattern on the transmission. Both had the standard 1 down 5 up pattern. On MT125R versions the shift lever was flipped over rearward to accommodate the rearset footpegs of the MT roadracer. To maintain the widely used 1 down 5 up pattern a different shift linkage was used behind the engine cover.

MT125Rs used a different cylinder than the CR versions when they were originally produced. At some point the part numbers were superseded by part numbers shared by both the MT125R and the CR125m Elsinore models.

Owners in search of parts after production stopped often substituted complete CR125M motors for their MT125R motors. The first clue is a reversed shift pattern from a stock MT125R (1 up 5 down).

Performance and riding the MT125R

Dyno test of Honda MT125 R-3

RPM	Standard HP	Standard torque (lb/ft)	Standard speed (6th gear)	Modified speed (6th gear)	Modified HP	Modified torque (lb/ft)
8500	16.67	10.3	88	88	14.16	8.75
9000	18.21	10.3	93	93	17.03	9.94
9500	19.9	11.0	98	98	19.54	10.80
10000	21.95	11.53	103	103	22.37	11.75
10500	24.39	12.20	108	108	25.01	12.51
11000	10.05	4.80	114	114	18.64	8.90
11250	n/a	n/a	n/a	116	17.07	6.57
11500	n/a	n/a	n/a	119	8.58	3.92

As with any 125cc roadracer, it was best suited for smaller riders. After bump starting the engine a good start required patience and finesse. To characterize the personality of the 125cc 2-stroke look at the RSC factory tachometer. It did not move or display engine rpm until 5,000 rpm and registered up to 14,000 rpm. With the tall gearing needed for top speed performance it would not pull its own weight until the tachometer was showing at least 9,000 rpm. The engine was in its peak powerband or, "on the pipe" from just below 10,000 rpm and went out of the powerband a little past 11,000 (depending on jetting and modifications).

To achieve a good start the rider needed to hold the throttle wide open and begin feathering the clutch as fast as possible while keeping the rpm between 10,000 and 11,000 rpm. If the tachometer dropped below 10,000 RPMs the rider needed to pull the clutch in, rev the motor and begin the process again. If the clutch was engaged too quickly the engine could "bog" or even stall while the rest of the starting field rode away. Even with the lightest of riders, the MT125R required slipping the clutch for 40 or more feet. The rewards for a successful launch were full throttle with quick shifts (approximately every 1,000 rpm) up through the 6 gears. At the end of sixth gear the hard working 123cc engine would be pulling along a straight stretch between 110 and 120 mph.